Sleep detection using an adjustable threshold

ABSTRACT

Devices and methods for sleep detection involve the use of an adjustable threshold for detecting sleep onset and termination. A method for detecting sleep includes adjusting a sleep threshold associated with a first sleep-related signal using a second sleep-related signal. The first sleep-related signal is compared to the adjusted threshold and sleep is detected based on the comparison. The sleep-related signals may be derived from implantable or external sensors. Additional sleep-related signals may be used to confirm the sleep condition. A sleep detector device implementing a sleep detection method may be a component of an implantable pulse generator such as a pacemaker or defibrillator.

RELATED PATENT DOCUMENTS

This is a division of patent application Ser. No. 10/309,771, filed onDec. 4, 2002, to issue as U.S. Pat. No. 7,189,204 on Mar. 13, 2007, towhich Applicant claims priority under 35 U.S.C. §120, and which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to sleep detection and, moreparticularly, to detecting sleep by adjusting a sleep thresholdassociated with a first sleep-related signal using a secondsleep-related signal.

BACKGROUND OF THE INVENTION

Sleep is generally beneficial and restorative to a patient, exertinggreat influence on the quality of life. A typical night's sleep for anormal person begins with a sleep stage known as slow wave sleep (SWS)characterized by low frequency electroencephalogram (EEG) activity. Asthe person falls asleep, brain activity declines and there is aprogressive increase in the depth of sleep. At approximately ninetyminute intervals, sleep lightens and a sleep stage known as rapid eyemovement (REM) sleep is initiated. REM sleep is characterized by highfrequency EEG activity, bursts of rapid eye movements, skeletal muscleatonia, and heightened autonomic activity.

There are typically 4-6 REM periods per night, with increasing durationand intensity toward morning. While dreams can occur during either REMor SWS sleep, the nature of the dreams varies depending on the type ofsleep. REM sleep dreams tend to be more vivid and emotionally intensethan SWS sleep dreams. Furthermore, autonomic nervous system activity isdramatically altered when REM sleep is initiated.

In patients with respiratory or heart disease, the brain during sleepcan precipitate breathing disturbances, myocardial ischemia, orarrhythmia. Although REM sleep is a necessary component of normal sleep,serious consequences may be associated with both the increase inautonomic activity and the intense emotional responses that accompanydreaming in patients with cardiovascular disease or respiratorydisorders, for example.

Disruptions of the respiratory system during sleep may include theconditions of sleep apnea or sleep hypopnea. Sleep apnea is a seriousbreathing disorder caused by airway obstruction, denoted obstructivesleep apnea, or derangement in central nervous system control ofrespiration, denoted central sleep apnea. Regardless of the type ofapnea, people with sleep apnea stop breathing repeatedly during theirsleep, sometimes hundreds of times a night and often for a minute orlonger. Whereas sleep apnea refers to cessation of breathing, hypopneais associated with periods of abnormally slow or shallow breathing. Witheach apnea or hypopnea event, the person generally briefly arouses toresume normal breathing. As a result, people with sleep apnea orhypopnea may experience sleep fragmented by frequent arousals.

An adequate quality and quantity of sleep is required to maintainphysiological homeostasis. Prolonged sleep deprivation or periods ofhighly fragmented sleep ultimately will have serious healthconsequences. Chronic lack of sleep may be associated with variouscardiac or respiratory disorders affecting a patient's health andquality of life.

SUMMARY OF THE INVENTION

The present invention is directed to detecting sleep. In one embodimentof the invention, a device for detecting sleep includes a first sensorfor sensing a first sleep-related signal and a second sensor for sensinga second sleep-related signal, wherein the first and the secondsleep-related signals are indicative of sleep. A sleep detector coupledto the first and the second sensors is configured to adjust a sleepthreshold associated with the first sleep-related signal using thesecond sleep-related signal. The sleep detector detects a sleepcondition by comparing the first sleep-related signal with the adjustedthreshold. A component of one or more of the sleep detector, firstsensor, and second sensor is implantable.

In accordance with another embodiment of the present invention, a methodfor sleep detection involves adjusting a sleep threshold associated witha first sleep-related signal using a second sleep-related signal. Thefirst sleep-related signal is compared to the adjusted threshold andsleep is detected based on the comparison.

Yet another embodiment of the invention includes means for adjusting asleep threshold of a first sleep-related signal using a secondsleep-related signal, means for comparing the first sleep-related signalto the adjusted threshold, and means for detecting sleep based on thecomparison.

In a further embodiment of the invention, a method for detecting sleepincludes sensing a plurality of sleep-related signals. A relationship isdefined between at least two of the sleep-related signals, therelationship associated with sleep detection. Sleep is detected usingthe sleep-related signal relationship. At least one of the sensing anddetecting is performed at least in part implantably.

The above summary of the invention is not intended to describe eachembodiment or every implementation of the present invention. Advantagesand attainments, together with a more complete understanding of theinvention, will become apparent and appreciated by referring to thefollowing detailed description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a sleep detection device in accordance withan embodiment of the invention;

FIG. 2 is a partial view of one embodiment of an implantable medicaldevice that may be used for sleep detection in accordance with anembodiment of the invention;

FIG. 3 is a system block diagram of an implantable medical device withwhich sleep detection may be implemented in accordance with anembodiment of the invention;

FIG. 4 is a graph of blood impedance used in connection with sleepdetection according to an embodiment of the invention;

FIG. 5 is a flow graph illustrating a method of detecting sleepaccording to an embodiment of the invention;

FIG. 6 is a flow graph illustrating a method of detecting sleep using anaccelerometer and a minute ventilation sensor according to an embodimentof the invention;

FIG. 7A is a graph of an accelerometer signal indicating patientactivity over time that may be used to implement a sleep detectionmethod in accordance with an embodiment of the present invention;

FIG. 7B is a graph of a heart rate signal indicating patient activityover time that may be used to implement a sleep detection method inaccordance with an embodiment of the present invention;

FIG. 8 is a graph of a minute ventilation signal indicating patientrespiration that may be used to implement a sleep detection method inaccordance with an embodiment of the present invention; and

FIG. 9 is a graph illustrating adjustment of an accelerometer sleepthreshold using an MV signal in accordance with an embodiment of theinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings which form a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An adequate duration and quality of sleep is required to maintainsleep-related homeostasis. Prolonged sleep deprivation or periods ofpoor quality sleep ultimately will have serious health consequences. Todiagnose the reasons for sleep disturbances, people suffering from sleepdisorders may spend one or more nights in a sleep laboratory. In a sleeplaboratory, a patient is typically instrumented for data acquisition andobserved by trained personnel. Sleep assessment in a laboratory settingpresents a number of obstacles in acquiring an accurate picture of apatient's typical sleep patterns. For example, spending a night in asleep laboratory typically causes a patient to experience a conditionknown as “first night syndrome,” involving disrupted sleep during thefirst few nights in an unfamiliar location. Furthermore, sleeping whileinstrumented and observed may not result in a realistic perspective ofthe patient's normal sleep patterns.

Sleep quality assessments depend upon acquiring data regarding apatient's typical sleep patterns. An initial step to sleep qualityassessment is an accurate and reliable method for recognizing that apatient is asleep. Detecting the onset, termination, duration, stages,and quality of sleep experienced by a patient may be used in connectionwith the treatment of various conditions. For example, detection ofdisordered breathing during sleep may be helpful in deliveringappropriate therapy for patients suffering from sleep disorders rangingfrom snoring to sleep apnea. Furthermore, trending sleep data over along term, including number and severity of disordered breathingepisodes, arousal episodes or periods of disturbed sleep, may provideinsight into the emotional and physical health of a patient. Forexample, knowledge of sleep patterns may influence a number of aspectsof patient therapy including cardiac or respiratory therapy.

In the context of cardiac rhythm management (CRM) therapy, for example,it may be advantageous to regulate the lower rate limit of a pacemakerbased on recognition of sleep or non-sleep states. Adjustment of thelower rate limit to accommodate periods of sleep may improve the qualityof the patient's sleep in addition to lengthening battery life of theCRM device. Furthermore, arrhythmia therapy may be improved with sleeprecognition. The periods of arousal from REM sleep have been associatedwith an increased likelihood of arrhythmia for patients with heartdisease. Therefore, the ability to recognize sleep may enhance theability to predict and detect arrhythmias associated with sleep and toprovide anti-arrhythmia therapy during sleep.

Respiratory therapy may also be enhanced by a method for accuratelyrecognizing a sleep state. Sleep apnea treatments may include positiveairway pressure devices that supply a steady or adjustable flow of airto the patient during sleep, periodic electrical stimulation of thehypoglossal nerve to open the upper airways, and cardiac atrialoverdrive pacing to suppress sleep apnea events or awaken the patient toterminate an apneic event. Each of these methods, as well as methods fortreating respiratory disorders, may be improved by reliable detectionthat the patient is sleeping.

Various embodiments of the invention involve detecting sleep usingsignals associated with a condition of sleep. One embodiment of theinvention involves adjusting a sleep threshold associated with a firstsleep-related signal using a second sleep-related signal. The firstsleep-related signal is compared to the adjusted threshold and sleep isdetected based on the comparison. At least one of sensing thesleep-related signals, comparing the first sleep-related signal to thesleep threshold, and detecting sleep is performed at least in partimplantably.

Another embodiment of the invention involves defining a relationshipbetween two or more sleep-related signals. The relationship isassociated with sleep detection. Sleep is detected using therelationship. Sensing the sleep-related signals and/or detecting sleepis performed at least in part implantably.

Defining a relationship includes, for example, establishing a sleepcriterion associated with at least one of the sleep-related signals. Thecriterion may be, for example, a threshold or other index related to thecondition of sleep. Detection of sleep involves comparing the sleepcriterion to the state of one or more of the sleep-related signals.

According to one embodiment of the invention, the sleep-related signalsmay be derived from external or implantable sensors and analyzed by anexternal sleep detector. Some or all of the sensors may have remotecommunication capabilities, such as a wireless Bluetooth communicationstransmitter or transceiver, to link them to the sleep detector.

According to another embodiment of the invention, the sleep-relatedsignals may be derived from external or implantable sensors and analyzedby an implantable device. The sleep detector may be a component of adevice that also performs other functions, such as cardiac pacemaker ordefibrillation functions. Some or all of the sensors may be wirelesslycoupled to the implantable device by telemetry, for example.

According to an embodiment of the present system, methods of sleepdetection may be implemented in an implantable cardiac rhythm management(CRM) system configured as a dual chamber pacemaker device which mayoperate in numerous pacing modes known in the art. The systems andmethods of the present invention may also be implemented in varioustypes of implantable or external diagnostic medical devices including,for example, polysomnography devices, respiratory monitors, and cardiacmonitors. In addition, the systems and methods of the present inventionmay be implemented in a number of implantable or external therapeuticmedical devices such as continuous positive airway pressure (CPAP)devices or hypoglossal nerve stimulators.

FIG. 1 is a block diagram of a sleep detection device 100 that may beused to detect sleep in accordance with an embodiment of the invention.The sleep detection device includes a number of sensors 101, 102, 103,104 that sense sleep-related signals associated with sleep. Arepresentative set of sensed sleep-related signals associated with sleepinclude body movement, heart rate, QT interval, eye movement,respiration rate, transthoracic impedance, tidal volume, minuteventilation, body posture, electroencephalogram (EEG), electrocardiogram(ECG), electrooculogram (EOG), electromyogram (EMG), muscle tone, bodytemperature, time of day, historical sleep times, blood pressure, andpulse oximetry.

A first sleep-related signal derived from a sleep detection sensor 101is a signal associated with sleep that is compared to a sleep thresholdfor detecting the onset and termination of sleep. A second sleep-relatedsignal derived from a threshold adjustment sensor 102 is used to adjustthe sleep threshold. Although one sleep detection sensor and onethreshold adjustment sensor are shown in FIG. 1, any number ofthresholds or other indices corresponding to a number of sleep detectionsensors may be used. Furthermore, signals from any number of adjustmentsensors may be used to adjust the thresholds or indices of a pluralityof sleep detection signals. Additional sleep-related signals derivedfrom confirmation sensors 103, 104 may optionally be used to confirm theonset or termination of the sleep condition.

The sleep-related signals derived from the sensors 101, 102, 103, 104are received by a sensor driver/detector system 110 which includesdetection circuitry 121, 122, 123, 124. The detection circuitry 121,122, 123, 124 may include, for example, amplifiers, signal processingcircuitry, and/or A/D conversion circuitry for each sensor signal. Thesensor driver/detector system 110 may further include sensor drivecircuitry 111, 112, 113, 114 required to activate the sensors 101, 102,103, 104.

A sleep detector 130, according to certain embodiments, transmitscontrol signals to the drive circuitry 111, 112, 113, 114 and receivessignals from the detection circuitry 121, 122, 123, 124. The sleepdetector 130 may include a microprocessor controller 131 whichcooperates with memory circuitry 132 for implementing sleep detectionmethods of the present invention. The memory circuitry 132 may be usedto store program data to implement sleep detection, to store parametersassociated with sleep detection, such as a sleep threshold, or to storehistorical data regarding sleep onset and termination over a selectedperiod.

The sleep detector 130 is configured to compare the level of a firstsleep-related signal to a sleep threshold adjusted by a secondsleep-related signal and determine sleep onset or termination based onthe comparison. The sleep detector 130 may use one or more thresholds orindices associated with one or more sleep-related signals. In addition,the sleep detector 130 may use one or more sleep-related signals toadjust the sleep thresholds or indices. Furthermore, the sleep detector130 may confirm the onset or termination of sleep using an additionalnumber of sleep-related signals.

The sleep detector 130 may include output circuitry 133 forcommunicating various signals associated with sleep to another device,to other components of a sleep detection device, a data storage deviceand/or a display device. The signals associated with sleep may include,for example, a sleep detection signal, parameters associated with sleepdetection, such as a sleep threshold, and/or historical data relevant tosleep (e.g., historical sleep time data or an average of same which canbe used to establish a sleep threshold). The sleep detector maycommunicate with another device over a wired or wireless communicationchannel, for example.

The sensors 101, 102, 103, 104 may comprise implantable sensors orexternal sensors. In one embodiment, the sensors 101, 102, 103, 104 arecoupled to the sensor driver/detector circuitry 110 and thus to thesleep detector 130 through a wired connection. In another embodiment,the sensors 101, 102, 103, 104 and sensor driver/detector circuitry 110are incorporated into sensing devices that include wirelesscommunication capabilities, e.g., a Bluetooth transmitter ortransceiver, and may be coupled to the sleep detector 130 through awireless link. The sleep detector 130 and/or sensor driver/detectorcircuitry 110 may be incorporated into an implantable or externaldevice.

FIG. 2 is a partial view of one embodiment of an implantable medicaldevice that may be used for sleep detection in accordance with theprinciples of the invention. The implantable device illustrated in FIG.2 is a cardiac rhythm management (CRM) system that includes animplantable pacemaker 200 electrically and physically coupled to anintracardiac lead system 202. The intracardiac lead system 202 isimplanted in a human body with portions of the intracardiac lead system202 inserted into a heart 201. The intracardiac lead system 202 is usedto detect and analyze electric cardiac signals produced by the heart 201and to provide electrical energy to the heart 201 under predeterminedconditions to treat cardiac arrhythmias of the heart 201.

The CRM 200 depicted in FIG. 2 is a dual chamber device, capable ofsensing signals from the right atrium and right ventricle and providingpacing pulses to the right atrium and the right ventricle. Low energypacing pulses may be delivered to the heart to regulate the heart beator maintain a lower rate heart beat, for example. In a configurationthat includes cardioversion/defibrillation capabilities, high energypulses may also be delivered to the heart if an arrhythmia is detectedthat requires cardioversion or defibrillation.

The intracardiac lead system 202 includes a right ventricular leadsystem 204 and a right atrial lead system 205. The right ventricularlead system 204 includes an RV-tip pace/sense electrode 212 and one ormore electrodes 213, 214, 216 suitable for measuring transthoracicimpedance. In one arrangement, impedance sense and drive electrodes 216,214, 213 are configured as ring electrodes. The impedance driveelectrode 213 may be located, for example, in the right ventricle 218.The impedance sense electrode 214 may be located in the right atrium220. Alternatively or additionally, an impedance sense electrode 216 maybe located in the superior right atrium 220 or near the right atrium 220within the superior vena cava 222.

A two-electrode impedance sensing configuration is also possible,wherein the right ventricular lead system includes an impedance driveelectrode 213 and a tip electrode 212. In this configuration, the tipelectrode 212 may be used as the impedance sense electrode as well as acardiac sense/pace electrode. Other locations and combinations ofimpedance sense and drive electrodes are also possible.

The atrial lead system 205 includes an A-tip cardiac pace/senseelectrode 256.

In the configuration of FIG. 2, the intracardiac lead system 202 ispositioned within the heart 201, with a portion of the atrial leadsystem 205 extending into the right atrium 220 and portions of the rightventricular lead system 204 extending through the right atrium 220 intothe right ventricle 218. The A-tip electrode 256 is positioned at anappropriate location within the right atrium 220 for pacing the rightatrium 220 and sensing cardiac activity in the right atrium 220. TheRV-tip electrode 212 is positioned at appropriate locations within theright ventricle 218 for pacing the right ventricle 218 and sensingcardiac activity in the right ventricle 218.

Additional configurations of sensing, pacing and defibrillationelectrodes can be included in the intracardiac lead system to allow forvarious sensing, pacing, and defibrillation capabilities of multipleheart chambers. In one configuration, the right ventricular and rightatrial leads may include additional electrodes for bipolar sensingand/or pacing, for example. Further, the right ventricular and rightatrial leads may also include additional electrodes for cardioversion ordefibrillation.

In other configurations, the intracardiac lead system may have only asingle lead with electrodes positioned in the right atrium or the rightventricle to implement sleep detection and single chamber cardiacpacing. In yet other embodiments, the intracardiac lead system mayinclude endocardial leads that are advanced into the coronary sinus andcoronary veins to locate the distal electrode(s) adjacent to the leftventricle or the left atrium.

Other intracardiac lead and electrode arrangements and configurationsknown in the art are also possible and considered to be within the scopeof the present system.

Referring now to FIG. 3, there is shown a block diagram of an embodimentof a CRM system 300 configured as a pacemaker and suitable forimplementing a sleep detection methodology of the present invention.FIG. 3 shows the CRM 300 divided into functional blocks. It will beunderstood by those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged andimplemented. The example depicted in FIG. 3 is one possible functionalarrangement. The CRM 300 includes sleep detection circuitry 320 forreceiving sleep-related signals and detecting sleep in accordance withan embodiment of the invention.

In one embodiment, sleep detection circuitry 320 is incorporated as partof the CRM circuitry 310 encased and hermetically sealed in a housing390 suitable for implanting in a human body. Power to the CRM 300 issupplied by an electrochemical battery power supply 312 housed withinthe CRM 300. A connector block (not shown) is additionally attached tothe CRM housing 390 to allow for the physical and electrical attachmentof the intracardiac lead system conductors to the CRM circuitry 310.

The CRM circuitry 310 may be configured as a programmablemicroprocessor-based system, with circuitry for detecting sleep inaddition to providing pacing therapy to the heart. Cardiac signals maybe detected by the detector circuitry 360 and delivered to the pacemakercontrol system 350. Pace pulses controlled by the pacemaker control 350and generated by the pulse generator 340 are delivered to the heart totreat various arrhythmias of the heart.

The memory circuit 316 may store parameters for various deviceoperations involved in sleep detection and/or cardiac pacing andsensing. The memory circuit 316 may also store data indicative ofsleep-related signals received by components of the CRM circuitry 310,such as the impedance drive/sense circuitry 330, the cardiac signaldetector system 360, and the accelerometer 335.

The sleep detection circuitry 320 receives signals derived from thecardiac signal detector system 360, the impedance driver/detectorcircuitry 330 and the accelerometer 335 to perform operations involvingdetecting sleep onset and termination according to the principles of thepresent invention. Historical data storage 318 may be coupled to thesleep detection circuitry 320 for storing historical sleep related data.Such data may be transmitted to an external programmer unit 380 and usedfor various diagnostic purposes and as needed or desired.

Telemetry circuitry 314 is coupled to the CRM circuitry 310 to allow theCRM 300 to communicate with an external programmer unit 380. In oneembodiment, the telemetry circuitry 314 and the programmer unit 380 usea wire loop antenna and a radio frequency telemetric link to receive andtransmit signals and data between the programmer unit 380 and telemetrycircuitry 314. In this manner, programming commands and data aretransferred between the CRM circuitry 310 and the programmer unit 380during and after implant.

The programming commands allow a physician to set or modify variousparameters used by the CRM. These parameters may include setting sleepdetection parameters for use during sleep detection, such as whichsleep-related signals are to be used for sleep detection and thresholdadjustment, and the initial sleep detection thresholds. In addition, theCRM system 300 may download to the programmer unit 380 stored datapertaining to sensed sleep periods, including the amount of time spentsleeping, the time of day sleep periods occurred, historical data onsleep times, and the number of arousals during the sleep periods, forexample.

Signals associated with patient activity and posture may be detectedthrough the use of an accelerometer 335 positioned within the housing390 of the CRM 300. The accelerometer responds to patient activity andthe accelerometer signal may be correlated with activity level, workloadand/or posture. Signals derived from the accelerometer 335 are coupledto the sleep detection circuitry 320 and may also be used by thepacemaker circuitry for implementing a rate adaptive pacing regimen, forexample.

The impedance sense electrode 214, the impedance drive electrode 213,and the impedance driver/detector circuitry 330 are used to detect avoltage signal related to transthoracic impedance. The transthoracicimpedance measurement may be used to calculate various parametersassociated with respiration. Under the control of the sleep detectioncircuitry 320, the impedance driver circuitry 332 produces a currentthat flows through the blood between the impedance drive electrode 213and the can electrode 309. The voltage at the impedance sense electrode214 relative to the can electrode 309 changes as the transthoracicimpedance changes. The voltage signal developed between the impedancesense electrode 214 and the can electrode 309 is detected by theimpedance sense amplifier 334 located within the impedancedriver/detector circuitry 330 and is delivered to the sleep detectioncircuitry 320 for further processing.

The voltage signal developed at the impedance sense electrode 214,illustrated in FIG. 4, is proportional to the transthoracic impedance,with the impedance increasing during respiratory inspiration anddecreasing during respiratory expiration. The peak-to-peak transition ofthe impedance measurement, illustrated in FIG. 4, is proportional to theamount of air inhaled in one breath, denoted the tidal volume. Theimpedance measurement may be further processed to determine the tidalvolume, corresponding to the volume of air moved in a breath, or minuteventilation corresponding to the amount of air moved per minute.

In addition to impedance and accelerometer measurements, cardiac signalsindicative of heart rate or other cardiac functions may also be used inconnection with sleep detection. Turning back to FIG. 3, cardiac signalsare sensed through use of the RV-tip and RA-tip sense electrodes 212,256. More particularly, the right ventricle signal may be detected as avoltage developed between the RV-tip electrode 212 and the can electrode309. Right ventricle cardiac signals are sensed and amplified by a rightventricle V-sense amplifier 362 located in the detector system 360. Theoutput of the right ventricle V-sense amplifier 362 may be coupled, forexample, to a signal processor and A/D converter within the detectorsystem 360. The processed right ventricle signals may be delivered tothe pacemaker control 350 and the sleep detection circuitry 320.

Right atrium cardiac signals are sensed and amplified by a right atrialA-sense amplifier 364 located in the detector system 360. The output ofthe right atrium A-sense amplifier 364 may be processed by signalprocessing circuitry and received by the pacemaker control 350 and thesleep detection circuitry 320.

The pacemaker control 350 communicates pacing control signals to thepulse generator circuitry 340 for delivering pacing stimulation pulsesto the RV-tip and RA-tip electrodes 212 and 256, respectively, accordingto a preestablished pacing regimen under appropriate conditions.

FIG. 5 illustrates a method of detecting sleep according to principlesof the invention. A sleep threshold associated with a firstsleep-related signal is established. The sleep threshold may bedetermined from clinical data of a sleep threshold associated with sleepacquired using a group of subjects, for example. The sleep threshold mayalso be determined using historical data taken from the particularpatient for whom the sleep condition is to be detected. For example, ahistory of a given patient's sleep times can be stored, and a sleepthreshold can be developed using data associated with the patient'ssleep time history.

The first sleep-related signal is sensed 510. A second sleep-relatedsignal associated with sleep is sensed 520. The first and the secondsleep-related signals may be sensed from sensors implanted in thepatient, attached externally to the patient or located nearby thepatient, for example. The first and the second sleep-related signals maybe any signal associated with the condition of sleep, such as therepresentative sleep-related signals associated with sleep listed above.

The sleep threshold established for the first sleep-related signal isadjusted using the second sleep-related signal 530. For example, if thesecond sleep-related signal indicates a high level of activity that isincompatible with a sleep state, the sleep threshold of the firstsleep-related signal may be adjusted downward to require sensing adecreased level of the first sleep-related signal before a sleepcondition is detected.

If the first sleep-related signal is consistent with sleep according tothe adjusted sleep threshold 540, a sleep condition is detected 550. Ifthe first sleep-related signal is not consistent with sleep using theadjusted sleep threshold, the first and the second sleep-related signalscontinue to be sensed 510, 520 and the threshold adjusted 530 until acondition of sleep is detected 550.

In another embodiment of the invention, illustrated in FIG. 6, anaccelerometer and a minute ventilation sensor are used as first andsecond signals associated with sleep. A preliminary accelerometer signalsleep threshold is determined 610. For example, the preliminary sleepthreshold may be determined from clinical data taken from a group ofsubjects or historical data taken from the patient over a period oftime.

The activity level of the patient is monitored using an accelerometer620 that may be incorporated into an implantable cardiac pacemaker asdescribed above. Alternatively, the accelerometer may be attachedexternally to the patient. The patient's minute ventilation (MV) signalis monitored 625. The MV signal may be acquired, for example, using thetransthoracic impedance method described above using an implantablecardiac device. Other methods of determining the MV signal are alsopossible and are considered to be within the scope of this invention.

In this example, the accelerometer signal represents the sleep detectionsignal associated with the sleep threshold. The MV signal is thethreshold adjustment signal used to adjust the sleep threshold. Heartrate is monitored 630 in this example to provide a sleep confirmationsignal.

Threshold adjustment may be accomplished by using the patient's MVsignal to moderate the accelerometer sleep threshold. If the patient'sMV signal is low relative to an expected MV level associated with sleep,the accelerometer sleep threshold is increased. Similarly, if thepatient's MV signal level is high relative to an expected MV levelassociated with sleep, the accelerometer sleep threshold is decreased.Thus, when the patient's MV level is high, less activity is required tomake the determination that the patient is sleeping. Conversely when thepatient's MV level is relatively low, a higher activity level may resultin detection of sleep. The use of two sleep-related signals to determinea sleep condition enhances the accuracy of sleep detection over previousmethods using only one sleep-related signal to determine that a patientis sleeping.

Various signal processing techniques may be employed to process the rawsensor signals. For example, a moving average of a plurality of samplesof each sleep-related signal may be calculated and used as thesleep-related signal. Furthermore, the sleep-related signals may befiltered and/or digitized. If the MV signal is high 635 relative to anexpected MV level associated with sleep, the accelerometer sleepthreshold is decreased 640. If the MV signal is low 635 relative to anexpected MV level associated with sleep, the accelerometer sleepthreshold is increased 645.

If the sensed accelerometer signal is less than or equal to the adjustedsleep threshold 650, and if the patient is not currently in a sleepstate 665, then the patient's heart rate is checked 680 to confirm thesleep condition. If the patient's heart rate is compatible with sleep680, then sleep onset is determined 690. If the patient's heart rate isincompatible with sleep, then the patient's sleep-related signalscontinue to be monitored.

If the accelerometer signal is less than or equal to the adjusted sleepthreshold 650 and if the patient is currently in a sleep state 665, thena continuing sleep state is determined and the patient's sleep-relatedsignals continue to be monitored for sleep termination to occur.

If the accelerometer signal is greater than the adjusted sleep threshold650 and the patient is not currently in a sleep state 660, then thepatient's sleep-related signals continue to be monitored until sleeponset is detected 690. If the accelerometer signal is greater than theadjusted sleep threshold 650 and the patient is currently in a sleepstate 660, then sleep termination is detected 670.

The graphs of FIGS. 7-9 illustrate the adjustment of the accelerometersleep threshold using the MV signal. The relationship between patientactivity and the accelerometer and MV signals is trended over a periodof time to determine relative signal levels associated with a sleepcondition. FIG. 7A illustrates activity as indicated by theaccelerometer signal. The patient's heart rate for the same period isgraphed in FIG. 7B. The accelerometer signal indicates a period of sleepassociated with a relatively low level of activity beginning at slightlybefore 23:00 and continuing through 6:00. Heart rate appropriatelytracks the activity level indicated by the accelerometer indicating asimilar period of low heart rate corresponding to sleep. Theaccelerometer trends are used to establish a threshold for sleepdetection.

FIG. 8 is a graph of baseline trending for an MV signal. Historical dataof minute ventilation of a patient is graphed over an 8 month period.The MV signal trending data is used to determine the MV signal levelassociated with sleep. In this example, a composite MV signal using thehistorical data indicates a roughly sinusoidal shape with the relativelylow MV levels occurring approximately during the period from hours 21:00through 8:00. The low MV levels are associated with periods of sleep.The MV signal level associated with sleep is used to implement sleepthreshold adjustment.

FIG. 9 illustrates adjustment of the accelerometer sleep threshold usingthe MV signal. The initial sleep threshold 910 is established using thebaseline accelerometer signal data acquired as discussed above. If thepatient's MV signal is low relative to an expected MV level associatedwith sleep, the accelerometer sleep threshold is increased 920. If thepatient's MV signal level is high relative to an expected MV levelassociated with sleep, the accelerometer sleep threshold is decreased930. When the patient's MV level is high, less activity detected by theaccelerometer is required to make the determination that the patient issleeping. However, if the patient's MV level is relatively low, a higheractivity level may result in detection of sleep. The use of twosleep-related signals to adjust a sleep threshold for determining asleep condition enhances the accuracy of sleep detection over previousmethods.

Additional sleep-related signals may be sensed and used to improve thesleep detection mechanism described above. For example, a posture sensormay be used to detect the posture of the patient and used to confirmsleep. If the posture sensor indicates a vertical posture, then theposture sensor signal may be used to override a determination of sleepusing the sleep detection and threshold adjustment signals. Othersignals may also be used in connection with sleep determination orconfirmation, including the representative set of sleep-related signalsassociated with sleep indicated above.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of detecting sleep, comprising: sensing a plurality ofsleep-related signals; defining a relationship between at least two ofthe sleep-related signals, the relationship associated with sleepdetection; and detecting sleep using the sleep-related signalrelationship, wherein at least one of sensing, defining, and detectingis performed at least in part implantably.
 2. The method of claim 1,wherein defining the relationship comprises defining the relationshipbetween a patient activity signal and a respiratory signal.
 3. Themethod of claim 1, wherein defining the relationship comprises definingthe relationship between a cardiac signal and a respiratory signal. 4.The method of claim 1, wherein defining the relationship comprisesestablishing a sleep criterion.
 5. The method of claim 4, whereinestablishing the sleep criterion comprises establishing a sleepthreshold associated with one of the sleep-related signals.
 6. Themethod of claim 4, wherein establishing the sleep criterion furthercomprises modifying the sleep criterion.
 7. The method of claim 4,wherein establishing the sleep criterion further comprises adjusting thesensitivity of the sleep criterion.
 8. The method of claim 4, furthercomprising adjusting the sleep criterion associated with onesleep-related signal using another sleep-related signal.
 9. The methodof claim 1, wherein at least one of the sleep-related signals is anon-physiological signal.
 10. A sleep detection device, comprising:sensors configured to sense a plurality of sleep-related signals; and asleep processor coupled to the sensors, the sleep processor configuredto detect sleep using based on a relationship between at least two ofthe sleep-related signals.
 11. The device of claim 10, wherein the sleepprocessor is implantable.
 12. The device of claim 10, wherein the sleepprocessor is configured to determine the relationship between the atleast two sleep-related signals based on patient data.
 13. The device ofclaim 10, wherein the relationship is established based on clinicaldata.
 14. The device of claim 10, wherein the sensors include anaccelerometer configured to sense patient activity and transthoracicimpedance sensor configured to sense respiration.
 15. The device ofclaim 10, wherein the sensors include cardiac electrodes configured tosense cardiac electrical signals.
 16. The device of claim 10, wherein atleast one of the sensors is external of the patient.
 17. The device ofclaim 10, wherein the sleep processor is a component of an implantablecardiac therapy device.
 18. A sleep detection system, comprising:sensors configured to detect a plurality of sleep-related signals; andmeans for implantably detecting sleep based on a relationship between atleast two of the sleep-related signals.
 19. The system of claim 18,further comprising means for determining the relationship between the atleast two sleep-related signals.
 20. The system of claim 18, furthercomprising means for establishing a sleep criterion associated with atleast one of the sleep-related signals.
 21. The system of claim 20,further comprising means for modifying the sleep criterion.
 22. Thesystem of claim 20, further comprising means for adjusting the sleepcriterion associated with one sleep-related signal using anothersleep-related signal.